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Help with backfire

Discussion in 'The Garage' started by RedBrute, Jan 27, 2005.

  1. RedBrute

    RedBrute 1/2 ton status

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    I'm trying to trouble shoot my son's truck long distance (He is in Suffield,Connecticut going to Tech school) and need some help. The truck is a bone stock '79 k10, 350, 4bbl quadrojet, 350 auto, 203TC. The problem is when cruising along, it backfires thruogh the carb. Not a real big BOOM, just a slight pop. It doesn't do this while accelerating, or while pulling a long hill, just when cruising at 30-40 MPH in town, less at highway speeds but still does it occasionally. The engine has 98k on it, I have replaced the distributor (reman unit) cap, rotor, plug wires, plugs, set timing, ALL vacuum hoses, replaced the fuel filter and "autopsied" the old one (paper element) Filter had some fine black debris and tiny red particles but was otherwise not what I would consider bad. Also has new fuel pump. Tonight I had him spray the carb base gasket with carb cleaner while it was running, idle speed didn't change at all. (I was hoping for a leaky base gasket) The carb is one of the "smog" ones that doesn't have the idle mixture screws, just plugs where the screws should be so I can't have him adjust mixture at idle. He has run 3 cans of drygas, 1 can of seafoam, and 1 bottle of STP fuel cleaner through it in the past 2 weeks. The problem has not changed at all from before, during, or after all of the above. I had him unplug the EGR valve vacuum line and plug it. I'm hoping that tommorow he'll call and tell me this helped but I'm running out of ideas. I think this is a "lean" condition causing the problem but obviously I'm unsuccesful so far . Anybody see anything I've overlooked or advise on what the problem may be. I'm thinking of a remanned carb next, but don't want to throw money at it beyond the basic tuneup parts I've done so far.
     
  2. diesel4me

    diesel4me 1 ton status Premium Member

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    sounds internal

    Looks like you have covered all the basics like timing,tune up,etc--I guess now it must be an internal problem,perhaps a valve spring is weak or broken,(usually shows up at high speed or under acceleration),or a leaky or burned valve(intake most likely,if it coughs back thru the carb)or maybe a cam lobe is going round on ya--if an exhaust valve lobe wears down,the valve cant open enough to let all the exhaust out,and it comes out the carb next time the intake valve on that cylinder opens...

    If it has HEI,it might be the module or pick up coil,or the ignition coil itself also--or maybe if the large red wire that powers the HEI has a bad connection and isnt delivering a full 12 volts to it,it could cause a misfire or backfire condition...
    Hopefully its something like that,easy to fix--not good weather for doing a cam swap or valve job in CT right now--I'm buried under 4 ft. of snow not that far from there..:crazy:
     
  3. sweetk30

    sweetk30 professional hooker Premium Member

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    dude where are you in sothern tier ny? drop me a email dude.

    and it may be the sound of a bad cam? the lobes may have gone flat on the exaust side. i got a bbc 454 that did that and it was the cam gone.
     
  4. RedBrute

    RedBrute 1/2 ton status

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    Thanks for the replies guys, I never thought about the cam, the problem has been getting worse as time goes on and at idle it skips sometimes and other times it idles smooth this could also be a vaccum leak. It's all a moot point right now, I'm driving to Connecticut tomorrow to leave him the wifes jeep grand cherokee and bringing the truck back ( I hope). He worked on it last night and thought he had it, but had trouble getting it started this morning, and left the lights on when he got to class so now a dead battery also. To SweetK30, I live just down 17 from you, Apalachin, Tioga county. I have to renew my membership so I can't PM you right now. Anyway, tomorrow should be interesting to say the least.:crazy:
     
  5. RedBrute

    RedBrute 1/2 ton status

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    So... went back thru all the vacuum lines, and all vacuum operated thingys. Found the EGR valve wouldn't hold vacuum, bad diaphram. Pulled the vac line off and plugged the line and took it for a drive. No more backfire. Pulled over on the way back and hooked the line back up and drove home...backfiring. For now I'll just leave it unhooked and plugged till the weather warms up enough to replace it.
     

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